The present invention relates to an electrophotographic image forming apparatus capable developing a latent image electrostatically formed on an image carrier with a dry toner to thereby produce a corresponding toner image.
An electronic copier, facsimile apparatus, printer or similar electrophotographic image forming apparatus having the above capability is operable at a high speed with plain papers and insures high image quality. An image forming process particular to this kind of apparatus is as follows. While an image carrier formed of a photoconductive material is rotated, a charger uniformly charges the surface of the image carrier. An exposing device scans the charged surface of the image carrier with a light beam for thereby electrostatically forming a latent image thereon. A developing unit develops the latent image with a charged toner and thereby produces a corresponding toner image. An image transferring unit transfers the toner image from the image carrier to a plain paper or similar recording medium. A fixing unit fixes the toner image on the paper by applying heat and pressure thereto. After the image transfer, a cleaning unit cleans or initializes the surface of the image carrier to prepare it for the next image formation.
Assume that the image carrier is uniformly charged to the negative polarity by the charger. Then, there are available a negative-to-positive and a positive-to-positive image forming process. In the negative-to-negative process, the exposing device illuminates the portions of the image carrier where the toner should be deposited. The developing unit charges the toner for developing the latent image to the negative polarity, while the image transferring unit charges the recording medium to the positive polarity. In the positive-to-positive process, the exposing device illuminates the background area of the image carrier. The developing unit charges the toner for developing the latent image to the positive polarity, while the image transferring unit charges the recording medium to the negative polarity. The negative-to-negative process and positive-to-positive process are executed mainly with a digital copier and printer and with an analog copier, respectively.
To clean the image carrier, the cleaning device has customarily been provided with an elastic blade or a fur brush, elastic roller or magnet brush which is rotatable. The blade is abutted against the surface of the image carrier so as to scrape off the toner remaining on the image carrier. The brush or the roller is rubbed against the surface of the image carrier while in rotation, thereby collecting the toner from the image carrier. Today, a toner having a small particle size and spherical configuration is available in order to enhance image quality. Because this kind of toner cannot be easily removed from the image carrier, use is often made of an electric bias, as disclosed in Japanese Patent Laid-Open Publication No. 5-61388 by way of example. An electric bias scheme taught in this document uses cleaning means implemented as a fur brush and reverses the polarity of the toner left on the image carrier by frictional charging.
On the other hand, there is an increasing demand for a toner recycling engine which saves natural resources and does not need maintenance. There has been proposed an engine including an exclusive path for conveying the toner from the cleaning unit to the developing unit. Japanese Patent Publication No. 61-3027 and Japanese Patent Laid-Open Publication No. 6-51672 each discloses an engine in which the toner removed from the image carrier by the cleaning means is again deposited on the image carrier and then collected by the developing device.
It is preferable to use a two-ingredient type developer, i.e., a toner and carrier mixture from the high-speed recording and stable developing process standpoint. The prerequisite with this kind of developer is that the ratio in weight of the toner to the developer, i.e., the toner concentration of the developer be controlled. Usually, a uniform latent image is formed in a part of the area of the image carrier from which the toner will not be transferred to the recording medium. This latent image is developed by the developing unit under constant conditions to turn out a reference toner image. A reflection density from the reference toner image is optically sensed by a density sensor. The output signal of the density sensor is used to control the replenishment of fresh toner into the developing unit or the developing conditions. For example, when the amount of toner forming the reference toner image is short of a preselected value, as indicated by the sensor output, fresh toner is replenished from a toner bottle into the developing unit. Although the toner concentration may be determined in terms of the transmissibility, electric capacity or inductance of the developer existing in the developing unit, this kind of approach lacks accuracy.
What is required of the image forming apparatus is a system which uses the toner and carrier mixture, allows the cleaning unit to remove the toner from the image carrier with an electric bias, and causes the developing unit to collect the removed toner. However, such a system brings about some problems ascribable to the reference toner image, as follows. Assume that when the toner collected by the cleaning unit is to be again deposited on the image carrier, the reference toner image arrives at the cleaning unit. Then, it is likely that the toner opposite in polarity to the toner in the developing unit remains on the image carrier without being collected by the cleaning unit and coheres on eh image carrier. Further, if the reference image is deposited on the cleaning unit, it is likely that the reference image is returned to the image carrier in the event of the next image formation, thereby smearing the resulting image. In addition, the toner of the reference toner image is apt to firmly adhere to the image carrier and thereby deteriorate the expected function of the image carrier. None of Japanese Patent Laid-Open Publication Nos. 5-61388 and Japanese Patent Publication No. 61-3027 mentioned earlier addresses to the above problems.